The present invention relates generally to the sealing of a small orifice in the presence of a high temperature, molten material and specifically to the use of boric oxide to temporarily seal an extrusion orifice in connection with the continuous casting of glassy metal alloy filaments.
Glassy metal alloys are of considerable technological interest owing to their extraordinary physical properties as compared to the properties characterizing the polycrystalline form of such alloys. An overview of the nature of such materials and their properties are given in "Metallic Glasses", 28:5 Physics Today (1975) by J. J. Gilman. The term "glassy metal alloy" is intended to refer to metals and alloys that are rapidly quenched from a liquid state to a substantially amorphous (noncrystalline) solid state, typically having less than about 50% crystallinity, and is considered to be synonymous with such terms as "amorphous metal alloy" and "metallic glass". Glassy metal alloys are well documented in the literature. For an extensive background see "Metallic Glasses", American Society for Metals (1978).
Glassy metal alloys are typically produced in filamentary form by extruding an appropriate molten alloy from a pressurized reservoir through an extrusion nozzle onto a high speed rotating quench surface, providing extreme quench rates typically exceeding 10.sup.5 .degree. C. per second, as is representatively shown in allowed U.S. Pat. Application No. 821,110 filed Aug. 2, 1977, now U.S. Pat. No. 4,142,571, for "Continuous Casting Method for Metallic Strips" by M. Narasimhan, hereby incorporated by reference. The term "filament" is intended to include ribbon-like strips, as well as wire-like filaments, of regular or irregular cross-section.
The present invention addresses the problem of the extrusion orifice becoming plugged in such operations, arising from reaction of the molten charge within the crucible with the ambient atmosphere during start up of the casting process. Typically the crucible is charged with the alloy to be extruded which is heated to the molten condition while concurrently producing a vacuum in the crucible sufficient to prevent the molten charge from flowing from the crucible through the extrusion orifice. When ready to cast, the crucible pressure is increased sufficiently to extrude the molten charge from the crucible through the extrusion orifice onto the rotating quench surface. During the holding period just prior to extrusion, the highly reactive molten metal has sufficient exposure to the atmosphere via the extrusion orifice such that certain alloying agents and trace-impurities are oxidized to form particulate matter in the molten charge as insoluble oxide particles. These oxide particles tend to accumulate in the extrusion orifice as casting proceeds or are taken into the cast filament as oxide inclusions, thus disturbing the uniformity of the cast filament.
This problem has been dealt with generally by providing an inert gas flow about the orifice. However, for configurations requiring that the outlet of the extrusion orifice be in close proximity to the rotating quench surface, the gas-cover apparatus must be removed from the vicinity of the nozzle as the crucible is brought onto the quench surface, thereby exposing the molten charge to oxidation for a brief but sufficient period to form particulate matter in the melt. A further step then has been to increase the temperature of the melt to solubilize these oxide particles. However, this procedure is highly undesirable since the melt must then be quenched over a greater temperature range. Practically, the extreme quench rates required in quenching the melt below its crystallization temperature to form an amorphous alloy are difficult to achieve.
An alternative approach to this problem is shown in U.S. Pat. No. 3,964,535 "Stopper Rod Tapping Assembly and Filament Forming Process" issued June 22, 1976, to J. Bedell and R. Smith, wherein a vented tapping rod effects closure and clearing of the extrusion orifice. While this approach is generally satisfactory, it requires the use of rather complex apparatus.
The present invention overcomes these limitations by providing an effective temporary seal for the extrusion orifice that is conveniently expelled from the orifice as extrusion is initiated with the leading surge of extruded molten alloy.